The catalytic converter in a motor vehicle is a part of its exhaust system and functions to decrease air pollution generated by such vehicles. In general terms, a catalytic converter includes a housing with an exhaust gas inlet at one end and an exhaust gas outlet at the other end. Within the housing, the exhaust gas contacts a catalyst which is carried on a support member capable of withstanding the temperature of the gas, i.e., as high as about 1200.degree. C. In order to withstand the heat, the support member is generally a monolithic ceramic honeycomb structure onto which the catalyst is applied. The ceramic materials employed are brittle, fragile and easily broken. Consequently, the catalyst support must be protected from excessive vibration and shock which could fracture it.
In order to cushion the catalyst support, a flexible, resilient intumescent mat material, having a nominal thickness between about 3 mm and about 12 mm and a density of about 0.3 to about 0.8 grams/cm.sup.3, is generally wrapped around the catalyst support, separating it from the housing wall. The first time the engine is run, the exhaust gas heats the intumescent mat, triggering its expansion to fill any void between the mat and the housing wall. The temperature at which intumescence occurs and the degree and permanence of the expansion attained are important parameters in this application. If the mat fails to expand properly at the exhaust gas temperature the catalyst support will not be effectively held in place.
As the exhaust gas initially enters the converter, the metal housing expands to a greater extent in response to the rising temperature than the ceramic catalyst support, creating an increasing gap between the two. The expansion of the intumescent mat must be both fast enough and large enough to tightly hold the catalyst support in place. The expanded mat also serves as a gas seal, preventing exhaust gas blow-by.
Diesel engines typically run at lower temperatures than gasoline engines, and the intumescent mat in a diesel engine catalytic converter is typically at a lower temperature than the corresponding intumescent mat in a gasoline engine, i.e., about 285.degree. C., versus about 600.degree. C. for a gasoline engine. The intumescent response of the mat in a diesel engine converter must be tailored to the lower temperature to ensure that the catalyst support is properly held in the catalytic converter and that exhaust gas blow-by is prevented.
The use of a firestop material to make a seal in openings through fireresistant building dividers is described in U.S. Pat. No. 4,363,199. The firestop material disclosed in the '199 patent includes ceramic fiber and a fire resistant molding compound and is not intumescent. An intumescent firestop material offers advantages, including a more effective seal against the walls of the opening. In such applications, the degree to which the intumescent mat expands is critical, for it must fill the space it is designed to occupy and must do so at a rapid rate. Intumescent response at a relatively low temperature, rapid rate of expansion, and a high degree of expansion are all desirable. A high degree of expansion ensures that the sheet material will be pressed firmly against the periphery of the opening to be sealed.
Intumescent mat materials typically have employed unexpanded micaceous minerals as the intumescent agent. For example, U.S. Pat. No. 3,916,057 and GB 1 513 808 disclose intumescent mat materials adapted for use in catalytic converters. The mat materials contain up to about 75 wt % to 85 wt % unexpanded vermiculite. The use of unexpanded vermiculite as the intumescent agent and expanded vermiculite as a filler in a mat material intended for catalytic converter applications is disclosed in U.S. Pat. No. 4,385,135.
In U.S. Pat. No. 4,305,992, it is noted that unexpanded vermiculite undergoes a "negative expansion," i.e., a contraction, as it is heated in the 300.degree. to 350.degree. C. temperature range, expanding only when heated to 375.degree. C. or higher. This initial contraction leaves the catalyst support open to damage unless and until the intumescent sheet expands sufficiently to bridge the space between the catalyst support and the housing wall. As disclosed in the '992 patent, treating the unexpanded vermiculite with an ammonium salt before using it reduces the initial contraction. This chemically treated (ion-exchanged) vermiculite is referred to as "IE vermiculite". The term "unexpanded vermiculite" is used herein to refer to either chemically treated or non-chemically treated vermiculite.
Although unexpanded vermiculite is commonly used as the intumescent agent in mat materials for catalytic converters, particulate graphite which has been treated with an oxidizing agent also becomes intumescent. The preparation of intumescent or expandable graphite is described in U.S. Pat. No. 4,454,190, where it is employed in making intumescent fiber felts for thermal insulation. The expandable graphite described in the '190 patent undergoes intumescence only when heated in the range 350.degree. C. to 600.degree. C., temperatures which are higher than desired for many applications, such as firestops or catalytic converters.
One advantage touted in the '190 patent for expandable graphite over unexpanded vermiculite as an intumescent agent is that the graphite burns off after it has caused the felt to expand. This "advantage" is, in fact, a disadvantage in other applications, e.g., catalytic converters.
In these applications, it is important that, once expanded in response to a sufficient temperature increase, the mat cannot shrink substantially if the mat is maintained at the increased temperature or cooled and then heated again repeatedly. With expandable graphite as the sole intumescent agent, it has been found that the mat shrinks upon continued heating at elevated temperatures, and the holding pressure of the mat against the catalyst support diminishes correspondingly.
The combination of vermiculite and graphite in a mat-like product has also been disclosed in the prior art. For example, a protective sheath or boot for electrical components is disclosed in U.S. Pat. No. 4,018,983. The product comprises a thermoplastic resin incorporating a heat-resistant fiber, which can be graphite fiber, and an intumescing or foaming component which can be vermiculite. A flexible mat useful as a gasket material or support, and containing exfoliated vermiculite together with graphite as a filler, is disclosed in U.S. Pat. No. 4,271,228. A mat material suitable for use in gaskets and containing both vermiculite and graphite flake is also disclosed in U.S. Pat. No. 4,443,517. A flexible mat material suitable for spiral-wound gaskets is described in U.S. Pat. No. 4,529,662, and the material can include mica or chlorite as well as graphite. A gel which can contain exfoliated vermiculite and graphite as a filler is disclosed in U.S. Pat. No. 4,676,929. Aside from U.S. Pat. No. 4,018,983, there is no indication that intumescence is a property of the articles containing both vermiculite and graphite which are described in the aforecited prior art.